The invention is directed to the preparation of polyenes belonging to the vitamin A and carotenoid series. In view of the extensive role that these materials play in the physical well-being of humans and animals and in the coloring of foods, there is a constant on-going effort to find methods of producing polyenes in an economical and efficient manner.
The invention disclosed and claimed herein employs 2-substituted-6-alkoxy-3,6-dihydro-2H-pyrans as starting materials to prepare polyene aldehydes. The 2H pyrans utilized herein are prepared by Wittig reactions on the products derived from the Diels-Alder reaction of 1-alkoxydienes and glyoxal or glyoxylic esters.
The preparation of simple 2-alkoxy-5,6-dihydro-2H-pyrans has been previously reported in the literature. Kubler, J. Org. Chem., 27, 1435 (1962), describes the preparation of such 2H dihydropyrans by the reaction of paraformaldehyde and 1-alkoxydienes. Shavrygina et al., J. Org. Chem., USSR, 2, 1393 (1966), disclose the preparation of certain 2-substituted-6-alkoxy-3,6-dihydro-2H-pyrans via the condensation of 1-alkoxydienes with paraform, chloral, and ethyl glyoxylate. The obtention of 6-alkoxy-3,6-dihydro-2H-pyran-2-carboxylic acid derivatives by the condensation of 1-alkoxydienes with esters of glyoxylic acid is disclosed by Konowal et al., Rocz. Chem., 42, 2045 (1968) and by Makin, Russ. Chem. Rev., 38, 237 (1969). There has been an extensive series of articles related to the use of 2-substituted-3,6-dihydro-2H-pyrans as starting materials for the synthesis of monosaccharides. Among such articles are Chimelewski et al., Rocz. Chem., 46, 627 (1972); Zwierzchowska et al., Rocz. Chem. 44, 1587 (1970).
In addition to the above-mentioned studies with 1-alkoxy-dienes, Zwierzchowska-Nowakowska et al., Rocz. Chem. 48, 1928 (1974), describes the Diels-Alder condensation of 1-acetoxy-1,3-butadiene with butyl glyoxylate to form acetoxy-substituted 2H dihydropyrans.
The article by Makin, (supra), discusses the syntheses of 1-alkoxydienes and their oligomerization to form polyenes. Particularly illustrated is the condensation of .alpha.,.beta.-unsaturated aldehydes with 1-alkoxydienes and subsequent conversion of the reaction products to polyenals. Compounds such as 5-ethoxy-citral, 5,9-diethyloxyfarnesal and others were obtained according to this method.
A mechanism for the condensation reaction between .alpha.,.beta.-unsaturated aldehydes and 1-alkoxydienes, proceeding via the intermediacy of a dihydropyran is proposed in Makin (by reference to an article by Krasnaya et al.), but the Krasnaya et al. article appearing in Zhurnal Obshchei Khimii, 30 (12), pp 3918-3926, 1960, indicates that the mechanism is not as proposed, and that dihydropyrans are not intermediates.
The preparation of polyene aldehydes of the vitamin A and carotene series via the reaction products of 1-alkoxydienes with glyoxal and glyoxylic esters, has been unknown heretofore.
The instant invention provides the art with a novel, efficient and economical method of preparing the above-mentioned polyene aldehydes.